134 SYDNEY J. HICKSON. 



The ectoderm is, then, a differentiation of the periphery of a 

 multinucleated plasmodium. 



What becomes of the inner part of the plasmodium ? 



We have no answer to this question so far as the Hydro- 

 corallines are concerned ; but, judging from the other Coelente- 

 rates, there can be little doubt, I think, that it becomes the 

 endoderm. 



In the development of Aglaophenia (Tichomiroff, 56) we 

 find a stage that is almost precisely similar to the solid planula 

 of Distichopora and Allopora, and in a later stage this central 

 yolk-laden plasmodium breaks up into blocks, which become 

 the endoderm-cells of the adult. 



The difficulty that we have now to face is, how can these 

 facts concerning the origin of the germ layers be brought into 

 line with those of other Coelenterates ? 



We find in the Stylasteridae no segmentation, no process of 

 invagination to form the endoderm, and no process that can be 

 compared with ordinary primary delamination ; but still it is 

 probable that this method of the formation of the germ layers, 

 if it is not itself the primitive one, has been derived from those 

 of other Coelenterates, and I shall endeavour to show in the 

 next section how the transition has taken place. 



II. On the Formation of the Germinal Layers in the 

 Ccelenterata. 



During the last ten years our knowledge of the early stages 

 of the development of the Ccelenterata has very considerably 

 widened, but still we seem to be no nearer to the solution of 

 many interesting phylogenetic questions than we were before. 

 The various theories that have been put forward, based upon 

 the study of a few forms, have in no instance received the un- 

 qualified approval of the principal authorities on the group, and 

 we find ourselves in a maze of conflicting theories, none of 

 which seem to conform entirely to our knowledge of facts. 



This unfortunate state of affairs is due to the fact that in the 

 group of the Ccelenterata we find many very diflFerent types of 



